Modem problems

Since yesterday I've been unable to get on the 'net reliably. Sometimes when I try to dial out XP just sits there claiming it's dialling indefinitely (no tone, dialog appears saying "Dialing <isp number>"). Sometimes, it does connect, but the PC immediately freezes (no keyboard or mouse response, hard reset required). And sometimes XP thinks I don't have a modem installed and the Modems Control Panel shows that my "Best Data Fax Modem" is not present.

A day earlier it was fine. Yesterday it worked fine once out of about 15 attempts at connecting.